Nanostructured materials (i.e., structures with at least one dimension in the range of 1-100 nm) have attracted steadily growing interest due to their unique, properties and potential applications complementary to three-dimensional bulk materials. Dimensionality plays a critical role in determining the properties of materials due to, for example, the different ways that electrons interact in three-dimensional (3D), two-dimensional (2D), one-dimensional (1D), and zero-dimensional (0D) structures. Compared with 0D nanostructures (so-called quantum dots or nano-particles) and 2D nanostructures (thin films), 1D nanostructures (including carbon nanotubes (CNTs) and nanowires (NWs)) are ideal as model systems for investigating the dependence of electronic transport, optical, and mechanical properties on size confinement and dimensionality as well as for various potential applications, including composite materials, electrode materials, field emitters, nanoelectronics, and nanoscale sensors.
Nanowires are a class of newer one-dimensional nanomaterials with a high aspect ratio (length-to-diameter typically greater than 10). They have interest separately from carbon nanotubes. Nanowires can be made of various compositions of materials in addition to carbon. Nanowires have demonstrated superior electrical, optical, mechanical and thermal properties. For example, the ultrahigh-strength of gold nanowires has recently been demonstrated. The significant increase in strength is due to reduced defects in the crystal structure and a smaller number of grains crossing the diameter of the nanowires. The broad choice of various crystalline materials and doping methods makes the properties (e.g. electrical) of nanowires tunable with a high degree of freedom and precision.
Nanowires consist of a variety of inorganic materials including elemental semiconductors (Si, Ge, and B), Group III-V semiconductors (GaN, GaAs, GaP, InP, InAs), Group II-VI semiconductors (ZnS, ZnSe, CdS, CdSe), and metal oxides (ZnO, MgO, SiO2, Al2O3, SnO2, WO3, TiO2). Among them, metal oxide nanowires have obvious advantages for some special applications due to their unique properties such as strong chemical interaction with metallic components. This phenomenon is sometimes explained as strong metal-support interaction. Significant progress has been reported in the use of metal oxide nanowires and nanobelts as sensors and in other electronic applications.
Substantially one-dimensional nanostructures (e.g. nanowires, nanorods, and nanobelts having a length much larger than thickness) are a new class of nanomaterials. Synthesis methods for such nanostructures usually fall into two categories: vapor-phase deposition or solution-based crystal growth. While solution-based syntheses generally offer better control of processing conditions and more easily achieve higher productivity, vapor deposition often yields higher aspect ratios (for example, length-to-width or length-to-diameter ratios) and excellent crystallinity due to the higher growth temperatures. However, one of the prominent current challenges is in controlling the synthesis of metal oxide nanostructures in ways that allow variation in their morphology. This would permit exploration of different potential materials applications of the nanostructures as their shapes are changed.